Electron emitter, manufacturing method thereof and electron beam device

ABSTRACT

High density of an electron emission at a low applied voltage is achieved for electron emitters and various products utilizing the emitters by hydrogenating lattice carbons of graphite crystallite of carbon nanotube or carbon film and thus forming &gt;CH— bonding group.

DETAILED DESCRIPTION OF THE INVENTION

[0001] The present invention relates to an electron emitter of carbonmaterials such as carbon nanotube, graphite and carbon film arrayed onan electron emission surface.

BACKGROUND ART

[0002] Carbon nanotube is used as a field emission type electron emitteran electron emitter for Field Emission Display (FED) as disclosed inJP-A-09-274844, JP-A-10-208677 and JP-A-11-306959 and as an electronemitter for fluorescent character display tube as shown inJP-A-11-162333 and Japanese Journal of Applied Physics, 37, L346 (1998).

[0003] An electron emitter using carbon nanotube with a tubularlyarrayed structure of carbon atoms has a diameter of several tens ofnanometer level, and carbon nanotube of electroconductive type enablesto take out a high emission current density at a lower extractionvoltage due to a far smaller radius of tip curvature in comparison witha conventional electron emission sources such as an electron emitterusing circular cone protrusions, when used as a field emission typeelectron emitter to take out electrons by applying an electric field. Inaddition, carbon can provides a longer life emitter because it does notmelt due to its high melting point, differing from metals.

[0004] However, as carbon materials such as graphite as a typical onehave work function values around 4.5 which are similar levels as oftungsten and the like, they have not necessarily realized sufficientlowering of an extraction voltage to the requirement to lower thevoltage in a driving circuit system, when used as an electron emitterarray. Furthermore, carbon nanotube has a feature that it essentiallyhas a semi-metallic nature and its electric conductivity largely dependson completeness of a graphite crystal structure, although a so-calledmultilayer carbon nanotube in which many layers of graphite tubes have anesting structure each other is electroconductive. Therefore, when anelectron emitter is operated, in particular, under a medium degree ofvacuum, that is, in a residual gas atmosphere of not less than 10⁻⁵ Pa,there is a problem that residual gas ions ionized by emitted electronsbombard an emitter, destroy a crystal array of graphite composing theemitter, and thus result in a reduced electric conductivity anddeteriorated emission characteristics.

[0005] On the other hand, another method already developed is to usetwo-dimensionally arrayed conical protrusions, that is, minuteprotrusions of circular cone or pyramidal shapes, as a field emissiontype electron emitter. As a method for forming protrusions, there areetching method of silicon and the like, transfer method of CVD diamondand the like or so-called Spint method in which conical protrusions areformed by a vapor deposition of molybdenum or the like through micropores, as disclosed in U.S. Pat. No. 3,789,471. These electron emittersexcept for CVD diamond, however, are inferior in heat resistance incomparison with carbon-based electron emitters, and thus are liable toincur an erosion by an electric discharge and do not have a sufficientlylong term reliability. A CVD diamond has also a disadvantage of limitedapplication range for manufacturing process due to its strict conditionsfor film formation such as substrate temperature. Furthermore, theseconical electron emitters, in general, have a disadvantage that theycannot provide a sufficiently large emission current for an appliedvoltage due to their larger radius of curvature compared with that ofcarbon nanotube.

[0006] An object of the present invention is to provide a carbon-basedemitter and electron beam device using it which can stably provide highdensity of emission current at lower voltage even under a relativelymedium degree of vacuum atmosphere.

MEANS FOR SOLVING THE PROBLEMS

[0007] Taking into consideration of the above problems, an embodiment ofthe present invention is an electron emitter having a >CH— bonding groupconsisting of a carbon atom linked to three neighboring carbon atoms andone hydrogen atom linked to said carbon atom. That is, the structure hashydrogen atoms of not only >CH₂ but also >CH— bondings arranged indefects or edge parts of graphite crystallites composing electronemitting surface or a layer just under the surface of an electronemitter. This structure enables to take out a far higher emissioncurrent than the structure without such hydrogen atom under the sameelectric field potential, or dramatically reduce an electric fieldpotential required for obtaining a prescribed level of current.

[0008] Another embodiment of the present invention is an electronemitter in which a film having carbon atoms is formed on anelectroconductive core protrusions, wherein said carbon atoms are thoseof >CH— bonding group consisting of a carbon atom linking to threeneighboring carbon atoms and one hydrogen atom. This emitter has apillar part of a composite structure of metal and graphite layer, whichelectrically connects a tip part of the electron emitter and substrateelectrode, and thus enables to solve the problem of an emission decreasephenomenon caused by an ion bombardment under a low degree vacuum regiondue to a semi-metallic nature of the conventional graphite.

[0009] According to the embodiments of the present invention, there canbe provided an electron emitter which has a far higher brightness incomparison with the conventional emitters and can stably work even undera medium degree vacuum region. Use of this emitter as an electron sourceenables a compact electron beam device with low energy consumption andhigh performance.

BRIEF DESCRIPTION OF THE DRAWINGS

[0010]FIG. 1 is a drawing comparing an electron emitter made ofmonolayer carbon nanotube according to the present invention and thatmade of a conventional monolayer carbon nanotube.

[0011]FIG. 2 is a graph showing a relation between an emission currentand a fraction of >CH— group of a hydrogenated multilayer carbonnanotube according to the present invention.

[0012]FIG. 3 is a longitudinal cross sectional drawing showing a crystalstructure of capped carbon nanotube treated with hydrogen according tothe present invention.

[0013]FIG. 4 is measurement results of Fourier Transform Infra-Redspectra of a hydrogenated multilayer carbon nanotube according to thepresent invention.

[0014]FIG. 5 is a graph showing a dependency of a field emission currenton an extraction voltage for a hydrogenated multilayer carbon nanotubeaccording to the present invention.

[0015]FIG. 6 is a graph showing a dependency of a field emission currenton an emission temperature for a hydrogenated multilayer carbon nanotubeaccording to the present invention.

[0016]FIG. 7 is a cross sectional structural drawing including a gatesubstrate, cathode, resistance layer and gate electrode of an electronemitter formed on a surface of circular cone core protrusions accordingto the present invention.

[0017]FIG. 8 is a cross sectional structural drawing of a carbon-basedelectron emitter formed on electroconductive needle-like coreprotrusions according to the present invention.

[0018]FIG. 9 is a cross sectional structural drawing of an electronemitter which is a carbon-based electron emitter with a coated metallayer on its external surface according to the present invention.

[0019]FIG. 10 is an example schematically showing a cross sectionalstructure of an image display device with two-dimensionally arrayedelectron emitters according to the present invention.

Numbers in these figures have the following meanings:

[0020]1: A carbon atom with three neighboring carbon atoms,

[0021]2: A carbon atom with two neighboring carbon atoms,

[0022]3: Hydrogen atoms, members of a >CH₂ bonding group linking to acarbon atom linked to the two neighboring carbon atoms among carbonatoms on an edge,

[0023]4: Hydrogen atoms, members of a >CH— bonding group, linking to acarbon atom linked to the three neighboring carbon atoms among carbonatoms on an edge,

[0024]5: A monolayer carbon nanotube composing a core of a cappedmultilayer carbon nanotube,

[0025]6: An axial part of a multilayer carbon nanotube,

[0026]7: A cap part of a multilayer carbon nanotube,

[0027]8: A hydrogenated layer in a hydrogenated multilayer carbonnanotube,

[0028]9: A deeper part without hydrogenation in a hydrogenatedmultilayer carbon nanotube,

[0029]10: Circular cone core protrusion,

[0030]11: Electron emitter layer,

[0031]12: Substrate,

[0032]13: Cathode,

[0033]14: Resistance layer,

[0034]15: Insulation layer,

[0035]16: Gate electrode,

[0036]17: Opening,

[0037]18: Focusing electrode,

[0038]19: Light transmission glass window,

[0039]20: Acceleration electrode,

[0040]21: Phosphor layer,

[0041]22: Side wall of a vacuum chamber,

[0042]23: Current introduction terminal for acceleration electrode,

[0043]24: Current introduction terminal for focusing electrode,

[0044]25: Current introduction terminal for gate electrode,

[0045]26: Current introduction terminal for cathode,

[0046]27: Electron beam,

[0047]28: Visible light,

[0048]29: Residual gas,

[0049]30: Carbon-based electron emitter having needle-like coreprotrusions,

[0050]31: Aluminum film,

[0051]32: Needle-like core protrusion,

[0052]33: Coated metal layer, and

[0053]34: Carbon nanotube.

DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION

[0054] Embodiments of the present invention will be explainedhereinbelow using drawings.

[0055] First of all, a graphite crystal structure of an electron emitterof the present invention is explained. FIG. 1 shows, as an example of anelectron emitter, bird's eye views comparing a crystal structure in thevicinity of the tip of a conventional monolayer carbon nanotube withouthydrogenation in FIG. 1(a), and a crystal structure of a monolayercarbon nanotube with hydrogenation according to the present invention atan edge part of the tip of the present invention in (b). The carbon atom1 in a graphite lattice at an end surface, that is, an edge part, of amonolayer carbon nanotube in FIG. 1 shows a carbon atom linked to threeneighboring carbon atoms among two bonding types of carbon atoms locatedon the edge. On the other hand, the carbon atom 2 shows a carbon atom onthe edge linked to two neighboring carbon atoms.

[0056] The hydrogenated carbon nanotube of the present invention shownin FIG. 1(b) has a structure in which at least a surface layer justunder an electron emission surface, that is an electron emitter layer,is composed of graphite crystallites with superior heat resistance andconductivity, whose defects, edge parts or five-membered ring parts arehydrogenated. The hydrogen atoms 4 show a hydrogen atom forming a >CH—bonding group linked to the carbon atom 1 shown in FIG. 1(a). And thehydrogen atoms 3 show a pair of hydrogen atoms forming a >CH₂ bondinggroup linking to the carbon atom 2 linked to two neighboring carbonatoms among the carbon atoms located on an edge of the carbon nanotube.

[0057] A high density of electron emission current can be provided byparticularly arranging hydrogen atoms that form a >CH— bonding group atleast on an electron emission surface as in an electron emitter of thepresent invention. Here, although only H is shown as hydrogen atom inthe drawing, formation of >CD— bonding group using D, that is deuterium,instead of H also provides similar effects.

[0058] Secondly, a hydrogenation treatment to form a >CH— bonding groupon an electron emission surface and a surface layer just under it of acarbon electron emitter is explained. Here, a method for producing ahydrogenated multilayer carbon nanotube according to this example willbe explained. Firstly, powder of multilayer carbon nanotube wasdissolved in a mixed solvent of cyclohexanone/toluene together with apolyurethane resin, followed by an ultrasonic treatment to obtain a welldispersed paste-like mixture. The paste-like mixture was then printed bysilkscreen printing on a nickel electrode with a pattern formed on aglass substrate. After that, the substrate was air dried, and afteroptionally forming gate electrodes, introduced into a preliminaryvacuuming room for degasing under a vacuum pressure of not higher than1×10⁻² Pa at 450° C. for about three hours. Subsequently, the substrate,after the vacuum degassing, was introduced into a plasma irradiationapparatus equipped with a plasma source of microwave-excited hydrogen,where hydrogen plasma was generated in a state in which hydrogen wasintroduced under a vacuum of 10⁻¹ Pa. Then hydrogen ions were irradiatedto the carbon nanotube electrode on the glass substrate for 20 minutesat a voltage of −150 V applied to the electrode on the glass substrate.The temperature of the substrate was maintained at 440° C. during theirradiation by heating a SUS table for mounting a substrate byelectrically heating a resistance type heater locating at the backsurface of the table. Thus, a hydrogenated layer with a >CH— bondinggroup was formed in an external circumferential surface of a multilayercarbon nanotube.

[0059]FIG. 3 shows a graphite crystal layer structure of the multilayercarbon nanotube thus obtained. FIG. 3 is a schematic drawing thereofshown by solid lines based on the results of a transmission electronmicroscopic observation. In the drawing, the hydrogen atoms linked to anedge part etc. of a graphite lattice are shown based on the results of aFT-IR (Fourier Transform Infra-Red Spectroscopy) analysis. Themultilayer carbon nanotube is a capped type multilayer carbon nanotubein which the multilayer carbon nanotube 6 has been formed around themonolayer carbon nanotube 5 by an arc discharge between graphiteelectrodes in helium gas of 0.5 atmosphere. The multilayer dome-like cap7 is similarly formed at the tip of 6. The hydrogenated layer 8 has beenformed in an external circumferential surface of the carbon nanotube bythe above described plasma treatment.

[0060] The results of FT-IR analysis revealed that the hydrogen atom 4belonging to the >CH— bonding group and the hydrogen atom 3 belonging tothe >CH₂ bonding group were bonded chemically to the graphite lattice.It was also found that the >CH— bonding group was linked chemically todefects or an edge surface in the graphite crystallites, or to thefive-membered ring lattice carbons in the dome-like cap part.

[0061] The results of electron microscopic observation also clarifiedthat the inter-layer spacing of graphite of the hydrogenated layers 8was expanded to 0.37-0.43 nm which was wider than the inter-layerspacing of graphite of the inner layer 9, d₀₀₂=0.34 nm. The expansion ofthe inter-layer spacing of the external layer 7 is caused by thepresence of C—H bondings, in particular, >CH— bonding groups. Here, theinter-layer spacing of graphite, d₀₀₂, means a distance between graphitelayer lattice planes. A multilayer carbon nanotube having a structuresimilar to the external layer 7 in FIG. 3 can be manufactured byhydrogen ion irradiation instead of hydrogen plasma. Further, it canalso be manufactured by an arc discharge evaporation deposition in ahydrogen atmosphere, or by irradiation of hydrocarbon-based materialswith electrons or rare gas ions.

[0062]FIG. 2 shows a relation between the field emission current at theextraction voltage of 900 V of the hydrogenated multilayer carbonnanotube electron emitter prepared according to this example and thefraction of the >CH— bonding group in various CH bonds determined fromabsorption intensities by stretching vibration of CH bonds using FourierTransform Infra-Red spectroscopy, so-called FT-IR method. The emissioncurrent by the field emission increases monotonically with the fractionof >CH— bonding group. When the fraction of >CH— bonding group is notless than 10%, the emission current increases by more than 100% ascompared with the case without hydrogenation. Furthermore, when thefraction of >CH— bonding group is not less than 20%, the field emissioncurrent increases by more than 200% as compared with the case withouthydrogenation.

[0063] Thus, according to the present invention, a dramatic enhancementof the field emission current can be attained by arranging >CH— bondinggroups on the electron emission surface. The fraction of >CH— bondinggroup in various C—H bonds is preferably not less than 10%, morepreferably not less than 20%.

[0064]FIG. 4 shows the measurement results of infra-red absorptionspectra (hereinafter the spectra) of C—H stretching vibration originatedby the presence of various C—H_(x) bonding groups of multilayer carbonnanotube irradiated with hydrogen ions at various irradiationtemperatures. Hydrogen ions H₃ ⁺of 1 keV was irradiated at the exposurerate of 1×10¹⁷ to 1×10¹⁸ H/cm² to a multilayer carbon nanotube withaverage diameter of 40 nm. The irradiated carbon nanotubes were mixedwith KBr powder and compressed to a pellet form, and the spectra in C—Hstretching vibrational infra-red absorption spectrum region weremeasured using a transmission type FT-IR (MODEL FTS-40A made byBio-Rad). The resolution setting was 0.4 cm⁻¹. Each peak of the spectrawas optimally and sequentially separated from each other approximatelyby a computer, and each peak component locating at the following wavenumber was separated: 3007±2 cm^(−1,) 3019±2 cm⁻¹ and 3031±2 cm⁻¹assigned to ═CH— bonding, 2957±4 cm⁻¹ assigned to nonsymmetricstretching vibration of —CH₃ bonding, 2925±3 cm⁻¹ assigned tononsymmetric stretching vibration of >CH₂ bonding, 2873±3 cm⁻¹ assignedto symmetric stretching vibration of —CH₃ bonding, 2855±3 cm⁻¹ and2840±3 cm⁻¹ assigned to symmetric stretching vibration of >CH₂ bondingand 2892±4 cm⁻¹ assigned to stretching vibration of >CH— bonding. Thedistribution function used for the peak separation is a mixed type ofGaussian and Lorentz distributions. The half peak widths were set at21-32 cm⁻¹ for nonsymmetrical stretching vibration of —CH₃ bonding,23-31 cm⁻¹ for nonsymmetrical stretching vibration of >CH₂ bonding,15-26 cm⁻¹ for symmetric stretching vibration of —CH₃ bonding, 15-23 cm¹for symmetric stretching vibration of >CH₂ bonding and 30 cm⁻¹ forstretching vibration of >CH— bonding. In the peak separation, the arearatio of symmetric stretching vibration to nonsymmetrical stretchingvibration of —CH₃ bonding was set to be in the range of 3.6-3.9, andsimilarly the area ratio of symmetric stretching vibration tononsymmetrical stretching vibration of >CH₂ bonding is in the range of2.1-2.8. The relative detection sensitivity ratio for the infra-redabsorption intensity of peaks assigned to each of the groups of ═CH—bonding, —CH₃ bonding, >CH₂ bonding and >CH— bonding is known to be0.12:2.2:1.1:1.0 from the measurement results of standard samples suchas cholesterol and menthol as shown in Journal of Nuclear Materials,266-269 (1999) 1051. Therefore, the relative densities of >CH— bondinggroup in the above four types of bondings can be estimated from theintegrated areas of the peaks assigned to the groups. The peak componentobserved at around 2890 cm⁻¹, shown with meshing in FIG. 4, is assignedto >CH— bonding group. The results of peak separation showed that theratio of >CH— bonding group to total CH_(x), bonding groups increasedwith the increase in the irradiation temperature when the irradiatedtemperature varies from the room temperature to about 450° C. butdecreased inversely at about 450° C. or higher. It is meaningless, inthis connection, to compare absolute peak intensities with those ofother spectra in the series of FT-IR spectra in FIG. 3, in considerationwith the measurement conditions, and it is important to compare theintensities among peak components within a same spectrum.

[0065]FIG. 5 shows the experimental results on extraction voltage andemission current characteristics of an electron emitter using themultilayer carbon nanotubes hydrogenated at various irradiationtemperatures as shown in FIG. 3, comparing with characteristics of acarbon nanotube ignition-treated under vacuum without a hydrogenation toeliminate adsorbed materials. It shows comparisons of electron emissioncharacteristics of electron emitters irradiated with hydrogen ionshaving an acceleration energy of 330 eV at room temperature, 440° C. and650° C. It is found that the electron emitters irradiated with hydrogenions have improved emission characteristics with a higher extractioncurrent (emission current) in any case in comparison with thenon-irradiated emitter. FIG. 6 also shows dependency of the emissioncurrent on the irradiation temperature at each extraction voltage basedon the data in FIG. 5. The emission current shows a tendency that itincreases with the increase in irradiation temperature of hydrogen ionsfrom room temperature up to around 440° C., but decreases at about 440°C. up to 650° C. This tendency is similar to the tendency of the effectof the irradiation temperature on the ratio of the >CH— bonding groupamong various C—H bondings. From these results, it becomes clear thatthe fraction of the >CH— bonding group depends on the irradiationtemperature and that the higher the fraction of >CH— bonding group, themore the emission characteristics is improved. As already describedabove, in the graph in FIG. 2 in which the electron emission current atthe extraction voltage of 900 V is plotted against the relative ratioof >CH— bonding group obtained from the results in FIG. 4, it is foundthat there is a positive correlation between the fraction of the >CH—bonding group and the emission current. On the other hand, anycorrelation was not observed between the relative densities of ═CH— or—CH₃ bonding group and the emission current. Also it is found there is anegative correlation between the fraction of >CH₂ bonding in the regionfrom room temperature to 450° C. and the emission current. Although anysample irradiated with hydrogen ions has more superior electron emissioncharacteristics than the non-irradiated samples, there is a tendencythat the lower the emission temperature the lower the emissioncharacteristics in the range of 100-450° C. This is because the ratio ofthe >CH— bonding group to other CH_(x) bonding groups, the formercontributing to the reduction of surface work function, varies inproportion to the ratio of graphite-like structure, and thus the ratiois higher as the irradiation temperature becomes higher in this range ofthe emission temperature. Taking into consideration of these results,the irradiation temperature ranges preferably 100-650° C., morepreferably 200-550° C.

[0066] As explained above, a high density emission current can beobtained at a low voltage by using a carbon nanotube having >CH— bondinggroups in which a carbon atom is linked to three neighboring carbonatoms and a hydrogen atom is linked to the carbon atom on an electronemission surface as an electron emitter according to this example.

[0067] In addition, because the carbon nanotube having hydrogenatedgraphite layers is modified with hydrogen in defects such as tip partswhich are intrinsically chemically reactive, it is chemically verystable and thus not oxidized even when left in air. Therefore, it canadvantageously provide very stable characteristics for an electronemitter or applications to other purposes. It also has an advantage ofan improved dispersion in non-polar solvents by the hydrogenation.Furthermore, it also has another advantage of a larger average distanceamong carbon nanotubes due to >CH₂ or >CH bonding groups saturated withhydrogen existing in defect parts at both ends or side surface,resulting in easier dispersion with little coagulation.

[0068] Next, another example of the electron emitter of the presentinvention will be explained.

[0069]FIG. 7 is a schematic drawing showing a cross sectional structureof a set of an electron emitter, gate electrode insulation layer andgate electrode, which set is a basic unit in applying the electronsource according to the present invention to FED, backlight for a liquidcrystal display and light source for a projection type display. Aresistance layer 14 is provided on a cathode 13 formed on a glasssubstrate 12, and a molybdenum gate electrode 16 having an opening 17 isformed on the resistance layer 14 by a lithography technique. Further, acircular cone or pyramidal core protrusions 10 made of molybdenum isformed coaxially with an opening of the gate electrode 14 by means ofSpint method, on which the outermost surface layer 11 of thehydrogenated carbon-based electron emitter is provided. Rough dimensionsof the circular cone core protrusions 10 is: 1 μm for diameter of thebottom, 1.3 μm for height, and the typical thickness of the electronemitter layer 11 is 100 nm. The method to form the electron emitterlayer: a carbon film having a thickness of 1-10 nm was firstly formed onthe surface of the circular cone core protrusions 10 by a sputteringmethod, followed by a hydrogen ion irradiation to the carbon film toproduce the electron emitter layer. The temperature of the substrate 12was kept at 450° C. during the irradiation of the hydrogen ions toattempt to optimize the film characteristics of hydrogenated graphitelayer structure. A hydrogen plasma irradiation can be used instead ofthe hydrogen ion irradiation, but even in this case, the substratetemperature during irradiation must be also at least not lower than 100°C., preferably not lower than 200° C., because the substrate temperatureduring irradiation lower than 100° C. gives amorphous carbon layersincluding insufficient graphite structure component, that is,insufficient electric conductivity, resulting in failing to fullyutilize the effects of the hydrogenation. In addition, the irradiationtemperature not lower than 650° C. reduces a concentration of hydrogenretained in the carbon layer to the level of not higher than 4% in H/Cratio, and cannot provide the hydrogenation effects sufficiently. Amagnified view of the cross sectional structure of the head part of theelectron emitter hydrogenated under the above-mentioned conditions isshown in the inserted drawing enclosed with a dot chain line in FIG. 7.Atomic array and state of hydrogen bonding in a graphite crystallitejust under an electron emission surface are schematically shown. Of thecarbon atoms on the edge, a carbon atom linked to two neighboring carbonatoms forms a >CH₂ bonding group by linking to two hydrogen atoms 3, onthe other hand, a carbon atom linking to three neighboring carbon atomsforms a >CH— bonding group by linking to one hydrogen atom 4, and thusthey contribute to the increase of electron emission amount. As the conecore protrusions other than this, silicon, niobium, nickel, tungsten,rhenium, iron, chrome, platinum and copper or alloys of two or moreelements thereof such as iron-nickel-chrome alloy, tungsten-rheniumalloy, nickel-chrome alloy and copper-nickel alloy can be used. Titaniumcarbide, titanium nitride and carbon as well as phenolic resins andpolyimide resins can also be used.

[0070] An advantage of the electron emitter of this example is that itpermits improving the heat resistance of emitters of conventional Spinttype or those formed by a lithography, and accordingly dramaticallyreducing the probability of erosion of the emitters duet arcing. Anotheradvantage is an effect to reduce threshold voltage for electron emissionof conventional electron emitters and thus reduce the drive voltage,that is, gate voltage for an on-off control of electron beams.

[0071]FIG. 8 is a conceptional drawing of an electron emitter showinganother example of the present invention. A carbon-based electronemitter layer is formed so as to cover the surface of needle-likeelectroconductive core protrusions oriented vertically on an electricresistance layer formed via a cathode on a glass substrate. As theneedle-like core protrusions, needle-like iron crystals with averagediameter of 30 nm and length of 400 nm are used. A cathode 13 is formedby printing a paste of tungsten fine powder on a substrate 12,thereafter a carbon film 14 is formed thereon by sputtering. Needle-likeiron core protrusions dispersed in an organic solvent is coated on thecarbon film 14 under a DC magnetic field applied in vertical directionto the substrate, so that they are oriented and fixed vertically to thesubstrate. In the same state, an impurity layer is eliminated from thesurface of the needle-like core protrusions using rare gas ions, whilesharpening the tip shape, then a carbon sputter layer with an averagethickness of 5 nm is formed by sputtering using a graphite target. By aseries of these processes, the needle-like metal core protrusions werecovered with carbon and fixed in such state as oriented nearlyvertically on the electrode. Then hydrogen irradiation was carried outby exposing the resultant protrusions to a hydrogen ion beam with anacceleration energy of 100 eV or to a hydrogen plasma at a minus biasvoltage applied to the substrate while keeping the substrate temperatureat 450° C. As a needle-like metal core protrusions other than iron whichcan be oriented by a magnetic field, chrome, cobalt, nickel and alloysof two or more metals thereof such as iron-chrome-nickel can be used.

[0072] As an alternative method for forming an electrode with verticallystanding, needle-like core protrusions, the following method can beused: A powder of an electroconductive needle-like core protrusions suchas iron having a graphite coating layer formed on its externalcircumferential surface is dispersed in a binder to give a paste, thenthe paste is coated on an electrode part on a substrate in a magneticfield. As a method for forming hydrogenated graphite layer on anelectron emission layer, a method of irradiating a substrate withhydrogen ion beam or hydrogen plasma while heating the substrate can beused. An advantage of the electron emitter prepared using theneedle-like metal core protrusions compared with the emitters preparedby the conventional methods such as lithography method, molding methodor Spint method is that the former enables to realize a larger area ofelectron emitter array at lower cost, since a printing method can beadopted.

[0073]FIG. 9 is a conceptional drawing of an electron emitter with anelectroconductive coating layer 33 formed on at least a part of sidesurface of the layers other than electron emitter layer of a carbonnanotube with a hydrogenated graphite tip. A carbon electrode 14 isformed via an electroconductive electrode 13 on the surface of a glasssubstrate 12, on which a carbon nanotube 34 is fixed using a binder.Subsequently the metal coating layer 33 with average thickness of 5 nmis formed by a vapor deposition of copper or nickel or by sputteringwith tungsten. Then the metal coating layer formed in the tip of theelectron emitter is removed as well as hydrogenation treatment ofgraphite by sputtering using hydrogen ion irradiation or hydrogen plasmairradiation in a state under a bias voltage applied condition. In thisprocess, because ion incident flux on a head part of the electronemitter is far higher than in a side surface, sputtering speed for themetal coating layer in the head part can be made faster than for theside surface, enabling selective removal of the metal coating layer inthe head part. The electron emitter thus formed can provide aconductivity by the metal coating layer formed in an externalcircumferential part, even if a conductivity of the carbon nanotube isdeteriorated by a bombardment of ionized residual gas going back throughan acceleration electric field against the electron emitter in a mediumdegree of vacuum region. Accordingly, the presence of anelectroconductive layer resistant to an ion bombardment retains theconductivity between the electron emitter layer as an electron emitterand the lower electrode 14, and thus makes it possible to suppress adeterioration of electron emission characteristics.

[0074] As another example, an electric emitter of the present inventionwas applied to an image display device. FIG. 10 is a conceptionaldrawing schematically showing a cross sectional structure cutperpendicularly to the scanning lines of a flat type display in whichthe electron emitters formed on needle-like core protrusions of thepresent invention are arranged like a two-dimensional array. Electronemitters 30 having a needle-like structure are arrayed in plane within agiven area as cathode elements, which are arranged like atwo-dimensional array. For each cathode element, a gate electrode 16 andfocusing electrode 18 made of molybdenum vapor deposition film areprovided between two-stage of insulation layer 15. A light transmittingglass window 19 is arranged in opposite to a substrate 12 by joining itto side walls 22 of a vacuum chamber made of glass. The whole vessel isair tightly sealed under an ultra high vacuum level. The major componentof residual gas 29 in the vacuum chamber is hydrogen and adjusted at notless than 1×10⁻⁶ Pa and not higher than 5×10⁻⁵ Pa in order to compensatehydrogen physically released from the electron emitter layer bybombardment of counter flow ions from the acceleration electrode, aswell as clean up the outermost surface of the electron emitter layer.The hydrogenation can lower the polarity of electron emitter surface andreduce the adsorption frequency of adsorbed polar molecules, and thusmakes it possible to enhance the stability of an electron emissioncurrent. On the other hand, if the hydrogen partial pressure exceeds theabove described range, wear of the electron emitter is accelerated bythe incidence of hydrogen ions ionized by the irradiation of electronbeam and raises a problem to shorten the lifetime of the emitter. Anacceleration electrode 20 is installed on an inner surface of the vacuumside of the light transmitting glass window 19, and a phosphor layer 21is formed on the acceleration electrode. An aluminum film 31 with athickness of 2 μm is deposited on a surface of phosphor layer 21 toprevent the decomposition of the layer by a bombardment of electronbeam, as well as to improve the utilization efficiency of the lightexcited in the phosphor layer by efficiently reflecting toward thedirection of the light transmitting glass window. In the side wall 22 ofthe vacuum glass vessel or on the substrate 12, a current introductionterminal for acceleration electrode 23, a current introduction terminalfor focusing electrode 24, a current introduction terminal for gateelectrode 25 and a current introduction terminal for cathode 26 aremounted and each of them is connected electrically with the accelerationelectrode 20, the focusing electrode 18, the gate electrode 16 and thecathode 13, respectively. The numbers of the gate electrode 16 and thecathode 13 mounted correspond to the number of pixels, and therefore thenumbers of the current introduction terminal for gate electrode 25 andthe current introduction terminal for cathode 26 mounted also correspondto the number of pixels. An electric field is generated at the tip ofthe electron emitter 30 by a high voltage from +6 kV to +10 kV appliedto the acceleration electrode, and the field emission electron 27 isemitted toward the acceleration electrode 20, focused by the focusingelectrode 18, transmitted through the aluminum layer 31 and enters intothe phosphor layer 21. The gate electrode is used to intercept anelectron beam by applying a negative gate voltage. When iron, cobalt oralloys thereof are used as the core protrusions 10 composing theelectron emitter 30, a divergence angle of emitted electrons can benarrowed and an improvement of brightness in the major axis directioncan be observed by forming the electron emitter with the coreprotrusions magnetized in its major axis direction in advance.

[0075] As described above, a high density emission current can beobtained at a low voltage by applying the hydrogenated electron emitterof the present invention to an electron emitters for the electron beamimage display device. In addition, the electron emitter can also providesuperior arc resistance and high reliability because the electronemitter surface is formed with carbon film or hydrogenated carbon film.

[0076] Although this example explains an embodiment using an electronemitter in which a hydrogenated carbon film is formed on a surface ofneedle-like core protrusions, the similar effects can also be obtainedby using the hydrogenated carbon nanotube according to the other exampleof the present invention or the electron emitters with a hydrogenatedcarbon film formed on the surface of the core protrusions formed bySpint method or a lithography method.

[0077] The electron emitter according to the present invention can beapplied to various electron beam application devices such as FED,backlight for liquid crystal display and flat type light source forprojection type display, and provides more compact devices with higherenergy efficiency and high performance. Furthermore, by applying theelectron emitter of the present invention, the following can beobtained: a fluorescent character display tube superior in luminousefficiency, a compact electron source for X-ray tube with low powerconsumption, an electron emitter for surge absorber, an electron sourcefor free electron laser, a compact power breaker for high voltage withhigh pressure resistance, a compact electron source for traveling wavetube superior in rapid start up and an electron tube for microwavegeneration.

[0078] The electron beam devices installing the electron emitter of thepresent invention enables to attain electron beam devices featuring inlight weight, compact, power saving and low cost due to the eliminationof a power source for heating cathode, which has been indispensable tothe electron beam devices using a conventional thermionic type electronsource, the simple structures and the rapid start up performance withoutpreheating. In particular, in electric beam devices requiring rapidswitching of an electron beam, any general purpose semiconductor circuitcan be used as a power circuit system for switching of an electron beam,and thus it has become possible to save costs in making a drive circuitsystem. Until now, many attempts have been made to field emission typeelectron sources using a metal electron emitter made of molybdenum,nickel or the like, but they still have problems of an erosion on thetip end by arc, high gate voltage due to large radius of the tip end,high manufacturing costs for a drive circuit system and a poor long termstability. The electron source according to the present invention cansolve all these problems. Moreover, it can also be effective inpreventing the deterioration in electron emission currentcharacteristics, which deterioration has caused a problem in using acarbon-based electron emitter, in particular, a carbon nanotube under amedium degree of vacuum region, and thus provide a practical electronsource with a superior long term reliability.

[0079] As explained above, the present invention can provide a highemission current at a low extraction voltage and attain an electronemitter with little deterioration in emission characteristics under amedium degree of vacuum level.

What is claimed is:
 1. A carbon nanotube characterized by comprisinga >CH— bonding group in which a carbon atom is linked to threeneighboring carbon atoms and one hydrogen atom is linked to said carbonatom.
 2. An electron emitter characterized by comprising a >CH— bondinggroup in which a carbon atom is linked to three neighboring carbon atomsand one hydrogen atom is linked to said carbon atom.
 3. An electronemitter characterized by comprising a >CD— bonding group in which acarbon atom is linked to three neighboring carbon atoms and a deuteriumatom is linked to said carbon atom.
 4. An electron emitter characterizedby having, within a C—H bonding stretching vibrational infra-redabsorption spectrum region, a peak component whose center of gravity islocated at 2892±4 cm⁻¹ corresponding to stretching vibration of >CH—. 5.An electron emitter characterized in that a fraction of >CH— bondinggroup relative to C—H bonding groups in ═CH— bonding, —CH₃ bonding, >CH₂bonding and >CH— bonding is at least 10%.
 6. An electron emitter using amultilayer carbon nanotube, characterized in that graphite crystallitesof said multiplayer carbon nanotube have an inter-layer spacing d₀₀₂ of0.37 to 0.43 nm.
 7. The electron emitter according to any one of claims1-4, wherein said electron emitter is a carbon nanotube.
 8. An electronemitter in which a film having carbon atom is formed on a surface ofelectroconductive core protrusions, wherein said carbon atom is a carbonatom linked to three neighboring carbon atoms, and composes >CH— bondinggroup in which one hydrogen atom is linked to said carbon atom linked tothree neighboring carbon atoms.
 9. An electron emitter characterized inthat a metal layer is formed on at least a part of side surface of acarbon nanotube.
 10. The electron emitter according to claim 9, whereinan electron emission surface of said carbon nanotube comprises >CH—bonding group in which a carbon atom is linked to three neighboringcarbon atoms and one hydrogen atom is linked to said carbon atom.
 11. Amethod for manufacturing an electron emitter characterized by comprisingthe step of forming hydrogenated carbon film having >CH— bonding groupon an electron emission surface of an electron emitter by irradiating acarbon-based or hydrocarbon-based material with hydrogen plasma orhydrogen ion at a temperature of 100° C.-650° C.
 12. The method formanufacturing an electron emitter according to claim 11, wherein saidtemperature ranges 300° C.-550° C.
 13. An electron beam device having avacuum chamber comprising the electron emitter according to any one ofclaims 2-10, an electron extraction electrode and current introductionterminals for providing a voltage to said electron emitter and saidelectron extraction electrode.
 14. The electron beam device according toclaim 13, wherein said vacuum chamber contains hydrogen in a pressureranging from 1×10⁻⁶ Pa inclusive to 5×10⁻⁵ Pa inclusive.